Because,
Tomcat, like all other J2EE/JEE webapps is a
web server, not a
file server.
You don't just plunk down files somewhere and expect them to work. You have to create a WAR.
A WAR is a JAR (zip) file with a certain required structure, the details of which I won't address here, since they're introduced early on in any book on Enterprise
Java webapps. Tomcat will accept WARs in either standard or unzipped ("exploded") forms, but the structure is the same, regardless.
The process of installing a WAR (and hence, a webapp) into Tomcat is known as "deploying", and it can be as simple as dropping a copy of the WAR file into the TOMCAT_HOME/webapps directory or as complex as constructing a sophisticated Tomcat Context file that points to a WAR in some arbitrary filesystem location. These methods, and quite a few more are described in detail in the documentation on the Tomcat website.
The documents also very strongly advice you
not to put Context elements in the Tomcat server.xml file. That's an ancient and obsolete practice and the only reason it still works is for special support that not even I have had to resort to (so far!)
Even after properly constructing a WAR and deploying it, to make a given resource render as the default application webpage requires that you define which resource(s) shall do so. That's defined in the webapp's WEB-INF/web.xml file. There are certain restrictions on what will and won't work as a default resource, however.
